Chapter 17: Adaptive Immunity: Specific Defenses of the Host 

17.1 The Adaptive Immune System

• Immunity to certain infectious diseases can be acquired through exposure which is known as adaptive immunity.

• Vaccination (immunization): a procedure that harnesses the adaptive immune response.

  • ==A vaccine formulated with a harmless version of a pathogen== incites an adaptive response which allows for people to become immune to illnesses without any danger of a full-blown infection.

• ==A crucial element is being able to differentiate between normal “self” cells and “nonself”.==

• The adaptive immune system comes into play only when innate defenses such as skin fail to stop a microbe.

  • Primary response: the first time the adaptive immune system meets and combats a particular antigen.
  • Later interactions with that same cell or substance will cause a secondary response.

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17.2 Dual Nature of the Adaptive Immune System

• ==Adaptive immunity is considered a dual system.==

• Humoral Immunity: immune actions taking place in these extracellular fluids, brought about by protective molecules called antibodies.

• Antibodies (immunoglobulin) combat foreign molecules called antigens.

• T lymphocytes or T cells are the basis of cellular immunity .

  • T cell receptors that recognize an antigenic peptide attached to a specialized presenting molecule on a cell.

• Cellular immune responses focus on recognizing antigens that have already entered a cell.

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17.3 Cytokine: Chemical Messengers of Immune Cells

• Cytokines are soluble proteins or glycoproteins.

  • ==Cytokines only acts on a cell that has a receptor for it.==

• Interleukins serve as communications primarily between leukocytes.

• Chemokines induce leukocytes to migrate to areas of infection of tissue damage where they can begin to act against an infection.

• Interferons were originally named for one of their functions: the ability to interfere with viral infections in host cells.

• Hematopoietic cytokines help control the pathways by which stem cells develop into red  blood cells or different white blood cells.

  • Overabundance of cytokines can do significant damage to tissues which appears as a factor in the pathology of certain diseases such as influenza.

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17.4 Antigens and Antibodies

• Substances that induce production of antibodies are called antigens.

  • Lipids and nucleic acids are usually antigenic only when combined with proteins and antibodies will react with the hapten independent of the carrier molecule.

• ==Antibodies are compact, relatively soluble proteins==

• The number of antigen-binding sites on an antibody is called the valence of that antibody.

• Monomer is a bivalent antibody that has the simplest molecular structure.

  • The two sections that are located at the ends of the Y arms are called variable regions.
  • The stem of the antibody monomer and the lower parts of the arms of the Y are called the constant regions.

• The simplest and most abundant immunoglobulins are minors and can also assume some different sizes and arrangements.

• The five classes of Igs are IgG, IgM, IgA, IgD, and IgE.

  • IgG accounts for about 80% of all antibodies in serum.
  • IgM makes up 6% of the antibodies in serum and has a pentamer structure.
  • IgA accounts for only about 13% of the antibodies in serum, but it is by far the most common form in mucous membranes and in body secretion such as mucus, salvica, tears, and breast milk.
  • IgD makes up only about 0.02% of the total serum antibodies.
  • IgE constitute only 0.002% of the total serum antibodies and bind tightly by their Fc regions to receptors on mast cells and basophils.

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17.5 Humoral Immunity Response Process

• ==Humoral immune actions take place in the extracellular spaces within the body.==

  • B cells selected to mature can be activated.

• Clonal expansion is the process that allows activated B cells to produce plasma cells that make antibodies as well as memory cells.

• An antigen that requires a type of  cell called a T helper cell to activate a B cell is known as T-dependent antigen.

  • B cells can be activated directly by some antigens, called T-independent antigens.

• When an antigen-presenting cell makes contact with an antigen that can combine with its particular reception, the APC and antigen bind.

• The APC displays digested antigen fragments on its surface by surface by combining them with its major histocompatibility complex.

• ==T-independent antigens stimulate B cells directly, without the help of T cells.==

• Harmful cells are usually eliminated at the immature lymphocyte stage by a process called clonal deleting.

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17.6 Results of the Antigen-Antibody Interaction

• When an antibody encounters an antigen for which it is specific, their binding forms an antigen-antibody complex.

  • The strength of the bond between antigen and antibody is called affinity.

• The antibody molecule itself is not damaging to the antigen.

• The binding marks foreign cells and molecules for destruction or neutralization by pahgytes and complement.

• In agglutination, antibodies cause antigens to clump together.

• Opsonization is the coating of antigens with antibodies or complement proteins.

• Antibody-dependent cell-mediated cytotoxicity resembles opsonization in that the target organism becomes coated with antibodies.

• Antibodies may trigger activation of the complement system.

• In neutralization, IgG antibodies inactivate microbes by blocking their attachment to host cells.

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17.7 Cellular Immunity Response Process

• Peyer’s patches are secondary lymphoid organs located on the intestinal wall.

• M cells take up antigens from the intestinal tract and allow their transfer to the lymphocytes and antigen-presenting cells of the immune system found throughout the intestinal tract.

• Antigen-presenting cells associated with cellular immunity include B cells, dendritic cells, and activated macrophages.

  • ==Dendritic cells have long extensions called dendrites.==

• Macrophages are usually found in a resting state.

• T cells are classified by certain glycoproteins on their cell surface called clusters of differentiation or CD.

  • T cells can recognize an antigen presented on the surface of a macrophage and activate it, making the macrophage more effective in both phagocytosis and in antigen presentation.

• T regulatory cells make up about 5-10% of the T cells population.

• Cytotoxic T lymphocytes are not capable of attacking target cells as they emerge from the thymus as cells.

• On the infected cell’s surface, they carry fragments of endogenous antigens that are generally synthesized within the cell and are mostly of viral or parasitic origin.

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17.8 Nonspecific Cells and Extracellular Killing by the Adaptive Immune System

• Natural killer cells can also destroy certain virus-infected cells and tumor cells and can attack parasites.

  • NK cells first contact the target cell and determine whether it expresses MHC class I self-antigens.

• WIth the help of antibodies produced by the humoral immune system, the cell-mediated immune system can stimulate natural killer cells and cells of the innate defense system, such as macrophages, to kill targeted cells.

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17.9 Immunological Memory

• Antibody-mediated immune responses intensify after the primary response, where a particular antigen is first met, and corresponding antibodies are produced.
• The secondary response is due to the portion of activated B cells that, instead of transforming into antibody-secreting plasma cells, become memory cells.
• The intensity of the antibody-mediated humoral response can be reflected by the antibody titer, the relative amount of antibody found in the blood serum. \n

17.10 Types of Adaptive Immunity

• Immunity is acquired actively when a person is exposed to microorganisms or foreign substances and the immune system responds.
• Naturally acquired active immunity develops from exposure to antigens, illness, and recovery.
  • Naturally acquired passive immunity is the transfer of antibodies from a mother to her infant.
• Artificially acquired active immunity is the result of vaccination.
  • ==Vaccines introduce antigens to the body.==
• Artificially acquired immunity involves the injection of antibodies into the body.
• When an individual is given artificially acquired passive immunity, it confers an immediate passive protection against the disease.

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